1. Field of the Invention
The present invention relates to plating apparatus and method which are preferably used for forming interconnects on a semiconductor substrate (including forming a seed layer, and forming a reinforcing seed layer on a seed layer for a purpose of reinforcing the seed layer), an interconnection-protective film, and a diffusion-preventive film.
2. Description of the Related Art
Conventionally, aluminum or aluminum alloy have been generally used as a material for forming interconnect circuits on semiconductor substrates. As integrated density has increased in recent years, there is a demand for employing a material having a higher conductivity as an interconnect material. Thus, there has been proposed a method in which copper or copper alloy is used as an interconnect material, and is filled into interconnect pattern trenches formed in a semiconductor substrate by plating the substrate.
There have been known various processes including CVD (chemical vapor deposition), sputtering, and the like to fill interconnect pattern trenches with copper or copper alloy. However, in a case where a metal layer comprises copper or copper alloy, i.e., a copper interconnect is formed, the CVD process is costly for forming the copper interconnect, and it is impossible for the sputtering process to fill copper or copper alloy into interconnect pattern trenches having a high aspect ratio, i.e., a high ratio of depth to width. A plating process does not have these drawbacks and is most effective to fill copper or copper alloy into interconnect pattern trenches.
On the other hand, instead of an electroless plating apparatus for performing electroless plating treatment by providing a plurality of units for carrying out a plating process, a pretreatment process associated with plating, and a cleaning process, there has been proposed an electroless plating apparatus for performing these various processes by a single unit. FIG. 10 is a schematic view showing this kind of electroless plating apparatus. As shown in FIG. 10, the elecetoless plating apparatus comprises a holding apparatus (holding device) 81 rotated by a motor M, and a cover 83 disposed around a periphery of a semiconductor substrate W placed on and held by the holding apparatus 81. In the electroless plating apparatus, while the semiconductor substrate W is rotated by the motor M at a position indicated by dotted lines in FIG. 10, a plating liquid is supplied from a plating tank 87 to a central upper surface of the semiconductor substrate W by a pump P. While the plating liquid is spreading on an entire upper surface of the semiconductor substrate W under centrifugal force caused by rotation of the semiconductor substrate W, for thereby plating the semiconductor substrate W, the plating liquid which has fallen from the semiconductor substrate W is returned from a plating liquid recovery section 85 of the cover 83 to the plating tank 87, and is then circulated.
On the other hand, the semiconductor substrate W which has been plated is lowered to a position indicated by solid lines in FIG. 10 and rotated at such position, and cleaning water is supplied to the semiconductor substrate W from a non-illustrated cleaning water supply apparatus (cleaning water supply device), whereby plating liquid is rinsed from the surface of the semiconductor substrate W, and used cleaning water is collected by a cleaning liquid recovery section 86 from which collected cleaning water is drained.
However, the above conventional electroless plating apparatus also the following various problems:    {circle around (1)} Because a plating liquid is always supplied onto a surface, to be plated, of a semiconductor substrate, a large amount of plating liquid is circulated and used. When a large amount of plating liquid is circulated and used, a large-sized pump is required. Thus, a liquid temperature maintaining apparatus which prevents a temperature rise of the liquid due to heat generation of the pump is needed to be provided, thereby increasing an apparatus cost and enlarging the apparatus, and hence increasing a cost for a clean room in which this apparatus is accommodated.    {circle around (2)} Because plating liquid is always circulated and used, by-products are accumulated in a system performing electroless plating, and a stable plating process cannot be maintained. In order to perform a stable plating process, an apparatus for analyzing and adjusting a plating liquid is needed to be installed, thus increasing an apparatus cost and a clean room cost.    {circle around (3)} Because a large amount of plating liquid is circulated and used, particles tend to be generated from respective components of the apparatus. Thus, it is necessary to install a filter F in a circulation path so as to increase an apparatus cost and a clean room cost.    {circle around (4)} When plating is performed in such a state that plating liquid is always supplied only to a certain portion of the surface to be plated, a thickness of plated film at the portion where the plating liquid has been supplied is smaller than a thickness of plated film at other portions, as confirmed by experiments. Thus, uniformity of a film thickness on the surface deteriorates. A reason for this is that flow rate and thickness of the plating liquid, and the like at the portion where the plating liquid has been supplied is different from those at other portions, and hence plating reaction states are different from each other.    {circle around (5)} In order to perform electroless plating, a temperature of a reaction surface between the surface to be plated and plating liquid is required to be maintained at a predetermined constant temperature. Therefore, a device for constantly heating a large amount of the plating liquid to a temperature optimum for a plating reaction is required to be provided, thus increasing an apparatus cost and a clean room cost. Further, since a temperature rise of plating liquid is necessary at all times, deterioration of the plating liquid is hastened.    {circle around (6)} A semiconductor substrate is always rotated, and hence heat dissipation due to peripheral speed of the semiconductor substrate causes a remarkable temperature drop, resulting in a failure to obtain a stable plating process.    {circle around (7)} In a case where a plating liquid is supplied to the surface to be plated not by dropping but by spraying, temperature control of the plating liquid becomes uncertain, and a stable plating process cannot be obtained.
Further, if an interconnect pattern having a uniform thickness should be formed on the semiconductor substrate by electroless copper plating, it is essential to maintain a plating temperature at respective plating reaction portions of the semiconductor substrate at a predetermined high temperature. In this case, FIG. 11 is a view showing measured results, obtained by experiments, of relationships between plating liquid temperature and film thickness in electroless copper plating.
Plating conditions in the experiments are as follows:
CD Specifications of the Plating Liquid
    CuSO4.5H2O 2.5 g/l    EDTA.2Na 20 g/l    NaOH 4 g/l    HCHO (37%) 5 ml/l{circle around (2)} Samples Used in Plating    Semiconductor substrate having a diameter of 8 inches wherein a barrier layer of TaN (20 nm) and a Cu seed layer (full plating) are formed on silicone.{circle around (3)} Plating Time    60 seconds
An electroless plating was applied to semiconductor substrates under the above plating conditions. As a result, as shown in FIG. 11, it was found that when a temperature of the plating liquid varied by 1° C. during a plating time period of one minute, a difference of film thickness amounted to approximately 1.8 nm, and as plating temperature increased, thickness of plated film increased. Therefore, as described above, in order to perform electroless plating so as to form a plated film having a desired thickness and a uniform thickness over an entire surface of the semiconductor substrate, the plating liquid must be maintained at a predetermined high temperature, and temperatures at respective portions of the semiconductor substrate must be controlled to be uniform during plating reaction.
Thus, in the conventional plating apparatus, there has been employed a mechanism for bringing a plating liquid, whose temperature has been increased in advance to a plating treatment temperature or higher, into contact with a surface, to be plated, of a semiconductor substrate and keeping a surface of plating reaction warm by virtue of a heater or a lamp.
However, in the conventional method, it has been difficult to keep the surface of plating reaction of the semiconductor substrate warm at a predetermined temperature accurately at all times during plating reaction. Particularly, in a case of a single wafer processing plating apparatus in which semiconductor substrates are plated one by one, a temperature of the semiconductor substrates must be increased one by one. In this case, it has been difficult to heat a semiconductor substrate to a predetermined temperature accurately and quickly and keep the semiconductor substrate at the predetermined temperature.